The present invention relates to a fixing device adapted for an image forming apparatus, such as an electronic copying machine.
In fixing devices used in image forming apparatuses, such as an electronic copying machine, a sheet to which an image composed of developer is transferred is interposed between a heating roller and a pressure roller, whereby the image is fixed on the sheet. The fixed sheet is then separated from the heating roller by separating means, and is guided toward a sheet discharge roller.
As is shown in FIG. 1, sheet 8, to which an image composed of developer 4 (hereinafter called "toner image") has been transferred in a developing section (not shown), is conveyed by conveyor belt 6, and is then guided by guide 10 into the nip portion between heating roller 12 and pressure roller 14. Sheet 8 is fed between rollers 12 and 14, to fix the toner image thereon, and is then separated from heating roller 12 by separation pawl 16.
Separating pawl 16 is rotatably connected to stationary frame 18 by shaft 20. End portion 16a of pawl 16 is biased by spring 22 and contacts the peripheral surface of heating roller 12. Pawl 16 has guide portion 16b for guiding sheet 8 separated by portion 16a from heating roller 12. Sheet 8 guided by guide portion 16b of pawl 16 is guided by conveyor guide 24 toward a pair of sheet discharge rollers 26.
End portion 16a contacts the periphery of roller 12 at position B (hereinafter called "contact point") remote from position A (hereinafter called "nip point") where heating roller 12 contacts pressure roller 14. Pawl 16 has bent portion 16c connecting end portion 16a and guide portion 16b.
Since nip point A and contact point B are remote from each other, the end portion of sheet 8, which has passed from nip point A, warps upwardly, along heating roller 12, then is separated from roller 12, then warps downwardly as it is guided along bend portion 16c of separation pawl 16, and is finally guided toward discharge rollers 26. As a result, entire sheet 8 assumes a wavy shape as is shown in FIG. 2. Wavy sheet 8 is undesirable and raises some problems when it undergoes two copying processes in a copying machine in which this fixing device 2 is used. First, when such wavy sheets 8 are bundled together, the resultant bundle is inevitably thick. Secondly, sheet 8, which has become greatly wavy during the first copying process as it passes by fixing device 2, is wrinkled during the second copying process. Thirdly, wavy sheet 8 cannot readily be separated from the photosensitive drum of the copying machine.